Question

在下面显示的代码中，蔬菜多次声明为常量。但是Xcode没有出错。为什么Xcode成功编译它并没有得到和错误？让我们不变。？

let vegetable = "red pepper"
switch vegetable {
case "celery":
    let vegetableComment = "Add some raisins and make ants on a log."
case "cucumber", "watercress":
    let vegetableComment = "That would make a good tea sandwich."
case let x where x.hasSuffix("pepper"):
    let vegetableComment = "Is it a spicy \(x)?"
default:
    let vegetableComment = "Everything tastes good in soup."
}

摘自：Apple Inc.“The Swift Programming Language。”iBooks。

Answer 1

我认为你实际上是在谈论vegetableComment被定义为一个常数，这就是你所困惑的。

在Swift中，case:语句中的每个switch块都有自己的词法范围。这意味着您可以在每个变量中将所有变量命名为相同，并且它们不会发生冲突。几乎就好像他们处于不同的职能部门一样。

同样，您无法访问其他变量。以下是一些例子：

let vegetable = "red pepper"
var comment = ""
switch vegetable {
case "celery":
    comment = "Add some raisins and make ants on a log."
    // This is only defined here
    var favoriteVegetable = "celery"

case "cucumber", "watercress":
    comment = "That would make a good tea sandwich."
    // This will be an error, because `favoriteVegetable` is only valid inside the celery case block
    // favoriteVegetable = "either cucumber or watercress"

case let x where x.hasSuffix("pepper"):
    comment = "Is it a spicy \(x)?"

    // We can redefine favoriteVegetable here, because it has nothing to do with the one in the celery block
    let favoriteVegetable = "a pepper"

default:
    comment = "Everything tastes good in soup."
}

// Similarly, we can't access `favoriteVegetable` here
// println(favoriteVegetable)

// This was defined before the switch statement, so we can get the value that was calculated
println(comment)

案例和swift切换

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